This invention relates to a frequency-division multiplexing transceive apparatus and method for sending and receiving data by a mobile-station-specific frequency spectrum. More specifically, the invention relates to a frequency-division multiplexing transceive apparatus and method for transmitting a transmit symbol upon subjecting the symbol to phase rotation that varies at a speed specific to a mobile station.
DS-CDMA (Direct Sequence-Code Division Multiple Access) multiplies a narrow-band transmit signal by a spreading code to thereby transmit the signal upon spreading it over a wider band. When each of a number of mobile stations sends a transmit signal upon multiplying it by a spreading code having a spreading factor SF in DS-CDMA, the information transmission speed is 1/SF. In order to achieve a frequency utilization efficiency equivalent to that of TDMA, therefore, it is required with DS-CDMA that the number of signals accommodated be equal to SF-number of mobile stations. However, in an actual radio propagation environment on the uplink, the influence of MAI (Multiple Access Interference), in which the signals from each of the mobile stations interfere with one another, becomes dominant owing to disparities in propagation conditions from each mobile station to the base station, e.g., disparities in propagation delay time or propagation-path fluctuation. The result is a decline in rate of frequency utilization.
IFDMA (Interleaved Frequency-Division Multiple Access) is being studied as a radio modulation scheme capable of diminishing the influence of MAI in next-generation mobile communications [see the specification of JP2004-297756 and “Investigations on Packet Error Rate of Variable Spreading and Chip Repetition Factors (VSCRF)-CDMA Wireless Access in Reverse Link Multi-cell Environment”, The Institute of Electronics, Information and Communication Engineers). This IFDMA modulating scheme transmits a transmit signal upon multiplying the signal by a phase the change in speed of which is specific to the mobile station, thereby reducing MAI by placing the signals from each of the mobile stations on a frequency axis in such a manner that the signals will not overlap one another on the frequency axis.
FIG. 16 is a block diagram illustrating the structure of a mobile station that employs an IFDMA modulating scheme, and FIG. 17 is a diagram useful in describing an IFDMA symbol. A channel encoder 1a performs channel encoding by applying error-correcting encoding such as turbo encoding or convolutional encoding to an entered binary information sequence, and a serial-to-parallel converter (a data modulator) 1b converts the channel-encoded data to, e.g., I, Q complex components (symbols) in QPSK by a serial-to-parallel conversion. A symbol transmitted in one frame of IFDMA is referred to as an “IFDMA symbol”. One IFDMA symbol is composed of Q-number of symbols S0, S1, S2 and S3 (Q=4 holds in this illustration), as illustrated at (a) of FIG. 17.
A symbol compression and repetition unit 1c compresses the time domains of the four symbols S0, S1, S2 and S3 that constitute the IFDMA symbol and repeatedly generates each symbol L times (L=4 in the illustration). In addition, the symbol compression and repetition unit 1c rearranges the repeatedly generated symbols and places them in the same arrangement as that of the symbol sequence S0, S1, S2, S3 [see (b) of FIG. 17].
One symbol that is the result of compressing the time domain is referred to as a sample (and is also referred to below as a symbol in the repetitive symbol sequence). If we let Tc represent the sample period, the period Ts of symbol repetition will satisfy the relation Ts=Tc×Q. A phase rotating unit 1d has a complex multiplier CML that subjects each symbol in the repetitive symbol sequence to user-dependent phase rotation [see (c) of FIG. 17]. A radio transmitter 1e up-converts the frequency of the signal, which enters from the phase rotating unit 1d, from baseband frequency to radio frequency, subsequently amplifies the radio-frequency signal and transmits the resultant signal from an antenna.
When the time domains of the transmit-symbol sequence S0, S1, S2, S3 are compressed, each transmit symbol is repeatedly generated a prescribed number (L) of times and the symbols of the repetitive symbol sequence are rearranged so as to have an arrangement identical with that of the symbol sequence S0, S1, S2, S3, the repetitive symbol sequence after rearrangement will have a a comb tooth-shaped spectrum, as illustrated at (a) of FIG. 18. If each symbol in the repetitive symbol sequence after the rearrangement is subjected to phase rotation that varies at a speed that is specific to the mobile station, the spectral positions of the a comb tooth-shaped spectrum will shift, as indicated at (a) to (d) of FIG. 18, and frequency-division multiplex transmission becomes possible. That is, if the speed of phase rotation is zero, the frequency spectrum of the output signal of phase rotating unit 1d will exhibit the a comb tooth-shaped spectrum characteristic shown at (a) of FIG. 18. As the amount of change (frequency) of phase rotation per unit time Tc increases, the frequency spectrum shifts as indicated at (a) to (d) of FIG. 18. It should be noted that W represents the symbol frequency.
An NCO (Numerically Controlled Oscillator) 1g calculates a phase rotation amount θ every unit time Tc, and the complex multiplier CML of the phase rotating unit 1d executes frequency shift processing by subjecting each symbol in the repetitive symbol sequence to phase rotation that is specific to the mobile station.
A phase θk(t) that is output from the NCO 1g when Q-number of symbols have been repeated L times is represented by the following equation:
                                                                                          θ                  k                                ⁡                                  (                  t                  )                                            =                                                                    k                    ·                    2                                    ⁢                  π                  ⁢                                                                          ⁢                                                            W                      L                                        ·                    t                                                  =                                                      k                    ·                    2                                    ⁢                  π                  ⁢                                                                          ⁢                                                            1                                              L                        ·                        Q                        ·                        Tc                                                              ·                    t                                                                                                                          W              =                                                                    1                    Ts                                    ⁢                                                                          ⁢                  QW                                =                                  1                  Tc                                                                                        (        1        )            where k, which represents a value that corresponds to the mobile station, is any one value among 0, 1, 2, . . . L−1. The NCO 1g outputs the phase θk(t), which has been calculated according to Equation (1), at the period Tc and is so adapted that the amount of phase rotation will be 2π at the IFDMA period (=16Tc) (i.e., such that the phase will make one full cycle).
The NCO 1g includes a frequency-shift setting unit 1h for setting the amount of change (angular speed) ω every unit time Tc. Using the parameters k, L and Q, the frequency-shift setting unit 1h calculates and outputs the angular speed ω according to the following equations:
                                                        ϖ              =                                                                    k                    ·                    2                                    ⁢                  π                  ⁢                                                                          ⁢                                      W                    L                                                  =                                                      k                    ·                    2                                    ⁢                  π                  ⁢                                                                          ⁢                                      1                                          L                      ·                      Q                                                                                                                                              f              =                                                ϖ                                      2                    ⁢                                          π                      ·                      Tc                                                                      =                                  k                                      L                    ·                    Q                    ·                    Tc                                                                                                          (        2        )            A rotation-phase amount deciding unit 1i in NCO 1g has an adder ADD and a delay unit DLY for applying a delay time T (=Tc). The deciding unit 1i performs a calculation according to the following equation every unit time Tc:θ(t+1)=θ(t)+ω  (3)increases the phase-rotation amount θ in increments of ω and outputs the result. A converter 1jcalculates I, Q components (x,y) in a complex plane of phase-rotation amount θ and inputs these components to the phase rotating unit 1d. If symbols constituting the repetitive symbol sequence are represented by S (=X+jY), then the phase rotating unit 1d performs a calculation according to the following expression:(X+jY)×(x+jy)and outputs the result of calculation. In actuality, the complex multiplier CML of the phase rotating unit 1d calculates and outputs (Xx−Yy), (Yy+Yx) for every real-number and imaginary-number part.
If k=0 holds, the amount f of frequency shift will be zero (f=0) and therefore the frequency spectrum will be as indicated at (a) of FIG. 18. If k=1 holds, then the amount f of frequency shift will be 1/(L×Q×Tc) according to Equation (2). If Q=L=4 holds, the phase changes in increments of π/8, as indicated at (c) of FIG. 19, and the frequency spectrum becomes as indicated at (d) of FIG. 19 or (b) of FIG. 18. As a result, even if a plurality of mobile station access the same base station simultaneously, the frequency spectrum of each mobile station will be orthogonal frequencies and interference among the transmit signals can be reduced.
FIG. 20 is another block diagram of a mobile station that employs an IFDMA modulating scheme, and FIG. 21 is a diagram useful in describing the operation of transmission with IFDMA modulation. The mobile station shown in FIG. 20 spreads transmit symbols by a spreading code, compresses and repeats the time domains of the spread chip sequence obtained by such spreading and rotates the phase of the obtained repetitive chip sequence every unit time Tc.
The channel encoder 1a performs channel encoding by applying error-correcting encoding such as turbo encoding or convolutional encoding to an entered binary information sequence, and the serial-to-parallel converter (a data modulator) 1b converts the channel-encoded data to, e.g., I, Q complex components (symbols) in QPSK by a serial-to-parallel conversion. One IFDMA symbol is composed of Q-number of symbols (Q=2 holds in this illustration), as illustrated at (a) of FIG. 21.
A spreading code multiplier 1m multiplies the symbols S0, S1 by spreading codes c00, c01, c10, c11 of spreading factor SF (SF=2 in this illustration) to thereby generate a spread chip sequence [(b) in FIG. 21]. As a result, one IFDMA symbol is composed of Q (=4) chips.
A chip compression and repetition unit 1n compresses the time domains of the four chips c00, c01, c10 and c11 that constitute the spread chip sequence and repeatedly generates each chip CRF times (CRF=4 in the illustration). In addition, the chip compression and repetition unit 1n rearranges the repetitive chip sequence and makes the sequence the same as the original chip sequence c00, c01, c10, c11 [see (c) and (d) of FIG. 21]. Here the number CRF of iterations stands for Chip Repetition Factor. If Tc represents the chip period, then the period Ts of the repetitive chip sequence is Tc×Q×SF.
The phase rotating unit 1d uses the complex multiplier CML to subject the repetitive chip sequence to user-dependent phase rotation [see (e) of FIG. 21]. The radio transmitter 1e up-converts the frequency of the input signal from baseband frequency to radio frequency, subsequently amplifies the radio-frequency signal and transmits the resultant signal from the antenna.
If the speed of phase rotation is zero, the frequency spectrum of the output signal of phase rotating unit 1d will exhibit a characteristic of the kind shown at (a) of FIG. 18. As the amount of change (frequency) of phase rotation per unit time Tc increases, the frequency spectrum shifts as indicated at (a) to (d) of FIG. 18.
The NCO 1g calculates the phase rotation amount θ every unit time Tc, and the complex multiplier CML of the phase rotating unit 1d executes frequency shift processing by subjecting each chip in the repetitive chip sequence to phase rotation that is specific to the mobile station. Phase θk(t) that is output from the NCO 1g when Q-number of chips have been repeated CRF times is represented by the following equation:
                                                                                          θ                  k                                ⁡                                  (                  t                  )                                            =                                                                    k                    ·                    2                                    ⁢                  π                  ⁢                                                                          ⁢                                                            W                      CRF                                        ·                    t                                                  =                                                      k                    ·                    2                                    ⁢                  π                  ⁢                                                                          ⁢                                                            1                                              CRF                        ·                        SF                        ·                        Q                        ·                        Tc                                                              ·                    t                                                                                                                          W              =                                                                    1                    Ts                                    ⁢                                                                          ⁢                                      SF                    ·                    Q                    ·                    W                                                  =                                  1                  Tc                                                                                        (        4        )            where k, which represents a value that corresponds to the mobile station, is any one value among 0, 1, 2, . . . CRF−1. The NCO 1g outputs the phase θk(t), which has been calculated according to Equation (4), at the period Tc and is so adapted that the amount of phase rotation will be 2π at the IFDMA period (=16Tc) (i.e., such that the phase will make one full cycle).
The frequency-shift setting unit 1h of the NCO 1g sets the amount of change (angular speed ω) every unit time Tc. Using the parameters k, CRF, Q and SF, the frequency-shift setting unit 1h calculates and outputs the angular speed ω according to the following equations:
                                                        ϖ              =                                                                    k                    ·                    2                                    ⁢                  π                  ⁢                                                                          ⁢                                      W                    CRF                                                  =                                                      k                    ·                    2                                    ⁢                  π                  ⁢                                                                          ⁢                                      1                                          CRF                      ·                      SF                      ·                      Q                                                                                                                                              f              =                                                ϖ                                      2                    ⁢                                          p                      ·                      Tc                                                                      =                                  k                                      CRF                    ·                    SF                    ·                    Q                    ·                    Tc                                                                                                          (        5        )            The rotation-phase amount deciding unit 1i, which has the adder ADD and the delay unit DLY for applying the delay time T (=Tc), performs a calculation according to the following equation every unit time Tc:θ(t+1)=θ(t)+ωincreases the phase-rotation amount θ in increments of ω and outputs the result. The converter 1j calculates I, Q components (x,y) in a complex plane of phase-rotation amount θ and inputs these components to the phase rotating unit 1d. The phase rotating unit 1d executes frequency shift processing by subjecting each chip of the repetitive chip sequence to phase rotation that is specific to the mobile station.
Thus, if k=0 holds, the amount f of frequency shift will be zero (f=0) and therefore the frequency spectrum will be as indicated at (a) of FIG. 18. If k=1 holds, then the amount f of frequency shift will be 1/(CRF×SF×Q×Tc) according to Equation (5). If Q=SF=2, CRF=4 holds, the phase changes in increments of π/8 and the frequency spectrum becomes as indicated at (b) of FIG. 18. Further, if k=2 holds, then the amount f of frequency shift will be 2/(CRF×SF×Q×Tc) according to Equation (5). If Q=SF=2, CRF=4 holds, the phase changes in increments of π/4 every Tc and the frequency spectrum becomes as indicated at (c) of FIG. 18. Further, if k=3 holds, then the amount f of frequency shift will be 3π/(CRF×SF×Q×Tc) according to Equation (5). If Q=SF=2, CRF=4 holds, the phase changes in increments of 3π/4 every Tc and the frequency spectrum becomes as indicated at (d) of FIG. 18. As a result, even if a plurality of mobile station access the same base station simultaneously, the frequency spectrum of each mobile station will be orthogonal frequencies and interference among the transmit signals can be reduced.
In the examples of the prior art, complex multiplication for performing phase rotation is carried out for every symbol of the repetitive symbol sequence or for every chip of the repetitive chip sequence in order to implement a frequency shift that is specific to the mobile station. As a consequence, a precise resolution is required for phase and a large amount of calculation is involved in phase rotation. For example, if the number of symbols constituting one IFDMA symbol is Q=4 and the number of iterations is L=4 in FIG. 16, or if the number of symbols constituting one IFDMA symbol is Q=2, and spreading factor is SF=2 and the number of chip iterations is CRF=4 in FIG. 20, then the resolution will be
                    θ        k            ⁡              (                  t          c                )              =                            k          ·          2                ⁢        π        ⁢                                  ⁢                  W          L                ⁢                  t          c                    =                        k          ·          2                ⁢        π        ⁢                                  ⁢                  1          16                ⁢                  t          c                      or                    θ        k            ⁡              (                  t          c                )              =                            k          ·          2                ⁢        π        ⁢                                  ⁢                  W          CRF                ⁢                  t          c                    =                        k          ·          2                ⁢        π        ⁢                                  ⁢                  1          16                ⁢                  t          c                    and 16 phase rotations will be required over the period of the IFDMA symbol (the frame period). The larger the number of iterations, the larger the number of symbols or the larger the spreading factor, the lower the resolution, the greater the amount of calculation and the greater the power consumption of the frequency-division multiplexing transmitting unit, e.g., mobile station. Although the foregoing is for the case of a transmitting unit, the amount of processing is great and the amount of power consumption large also in the case of a receiving unit.